The Res Gestae of Ammianus Marcellinus poses numerous structural puzzles for the historian, among them the anomalous final book, numbered 31 in the manuscript tradition. This book, which treats the Gothic rebellion of a.d. 376–378 and the campaign of Adrianople, is loosely connected to the other extant books, which conclude with events of a.d. 375. The present article argues that Book 31 was in origin a separate monograph, drafted in Greek at Antioch in the aftermath of the Roman defeat at Adrianople. Perhaps modelled on the Scythica of Dexippus, its contents reflect the Antiochene and Constantinopolitan polemic of its moment. For reasons that must remain speculative, Ammianus later translated his work into Latin and appended it to a finished draft of the Res Gestae.